The over 1.14 million soldiers in the United States Army are vital for national safety and security. Soldiers have unique physical demands due to engagement in combat and training operations. Maintaining and improving health, physical fitness, and body composition are imperative for soldier and are high priorities for the Department of Defense (DoD), as evidenced by the release of the Total Force Fitness (TFF) training model in 2010. However, overweight and obesity continue to increase among active duty soldiers, putting them at greater risk for significant health problems, activity restrictions, and impaired job performance, all of which result in decreased combat readiness. Current physical fitness standards in the Army Physical Readiness Training (APRT) program and the Army Physical Fitness Test (APFT) were judged inadequate for promoting soldiers' abilities to meet demands of combat and modern military operations because they fail to achieve important TFF fitness goals. Research on the best methods to achieve optimal physical fitness within the TFF paradigm is a top priority of the DoD health and fitness community. High-intensity functional training (HIFT), a very promising fitness approach, has gained popularity among military populations at the grass-roots level due to its potential for delivering improved fitness, including aerobic, anaerobic, and muscle endurance and greater strength outcomes with substantially lower training volumes. HIFT addresses and exceeds the fitness domains outlined in TFF and has the potential to address overweight and obesity among active duty military because of its potential for promoting fat loss. However, there is a lack of clear scientific evidence documenting the true effectiveness of HIFT training programs; to date, there have been no large randomized HIFT studies in any occupational group. In the proposed cluster-randomized clinical trial we will test the effectiveness of HIFT compared to usual care APRT in active duty military personnel. Unique strengths of this application include strong support from the Commanding General of the Combined Arms Center at Ft. Leavenworth (CACFL) and the US Army's Command and General Staff College (CGSC), access to a large population of soldiers for recruitment, and an internationally recognized scientific team with over 20 years of experience conducting high-impact obesity, physical activity, nutrition, and military health research. The effectiveness of this 6 month intervention will be determined by assessing changes in body composition (e.g., body fat percentage, weight), fitness (e.g., APFT, strength, agility), and health (e.g., cardiovascular health risk and systemic inflammation) between baseline and the end of treatment (6-months), as well as at 3-months follow-up (9-months). This application is innovative and significant given it addresses an important health concern in an essential occupational group and focuses on a training methodology with high potential for positively impacting our nation's safety and security. In addition, study results have implications for the training of other occupational groups which require healthy body composition along with a high level of fitness.